Thermal behavior of CMC solutions for continuous-flow radio frequency pasteurization

Abstract

Long heating time and non-uniform heating are critical issues for traditional thermal treatment to pasteurize and inactivate enzymes in liquid food, especially in high-viscosity liquid food. In this study, a carboxymethylcellulose (CMC) solution with high-viscosity non-Newtonian characteristics was selected as a model liquid food to explore the thermal behavior of continuous-flow based on a 6 kW, 27.12 MHz RF heating system used for a new physical pasteurization and enzyme-inactivation method. The established model for continuous-flow RF heating was verified by the temperature, residence time distribution, and cumulative functions of the CMC solution. The dimensionless analysis showed that free and forced convection had the same order of magnitude, indicating that the mixed convection occurred in the CMC solution. The final temperature of the solution increased with increasing solution concentration and decreasing volume flow rate. However, the fluid exhibited reciprocating motion and temperature fluctuations when the volumetric flow rate was 550 mL/min. The electric field intensity of the solution was higher at the inlet cross-section than at the outlet, but was the lowest in the middle one. The velocity of the solution increased with increasing volumetric flow rate and decreasing solution concentration. The temperature of the solution increased along the direction of liquid flow. In addition, the buoyancy effect caused high-temperature fluids to converge in the upper zone of the tube, leading to asymmetric velocity and inhomogeneous temperature distributions. The results of this study may provide valuable information for continuous-flow RF heating of non-Newtonian liquid and the potential industrial application of continuous RF pasteurization technology.